Thermochromic paints are used in a wide variety of applications, where it is desirable to monitor and control the temperature of a material through a heating process. One application for which thermochromic paints have been used, is in the heating of plastics to deform or shrink them with the assurance that the heating process will not cause permanent degradation of that particular plastic. One example for the use of thermochromic paints on plastic is on heat-shrinkable plastic sleeves which is used for connecting various types of conduits. The use of such heat-shrinkable sleeves are more fully described in U.S. Pat. Nos. 3,379,218 to Conde and 3,396,460 to Wetmore and assigned to the same assignee as the present invention. For this particular application, an irradiated thermoplastic, and often a polyolefin polymer or co-polymer which is subject to oxidative degradation is used. The heat-shrinkable plastic sleeve is placed over the conduits to be connected in an expanded state, and then heated, so it will shrink tightly over ends of the conduits, thereby connecting them.
For such connections, it has also been found beneficial to form a permanent bond between the heat-shrinkable material and the substrate over which it is shrunk. Thus, for many applications, an adhesive material is coated onto the interior walls of the heat-shrinkable sleeve so as to permanently bond the sleeve to the conduit. Adhesives which have been found particularly suitable for this type of application, are hot melt adhesives, as generally described in Lopez et al U.S. Application Ser. No. 291,542 filed Sept. 25, 1972 abandoned in favor of a continuation in part application which issued on Apr. 19, 1977 as U.S. Pat. No. 4,018,733; which is assigned to the same assignee as the present invention.
For this particular application, heat is required to both shrink the expanded plastic material and to melt the adhesive in order to permanently bond the plastic material to the substrate. It should be noted, that in many instances where this or a similar process is used to join conduits such as piping or electrical cables, it is under field conditions such as in a mine or on telephone lines. The heating unit may simply be a torch which will readily degrade the plastic if the temperature of the plastic is not monitored or controlled.
Thus, the use of thermochromic paints on the outer surface of a heat-shrinkable plastic has been found advantageous for indicating when a sufficient temperature has been reached to melt the adhesive on the interior surface, thereby bonding the plastic to the substrate, while assuring that the temperature of the plastic has not overheated to cause degradation. Various thermochromic paints, with different thermochromic pigments may be applied so as to change colors through a given temperature range. Many thermochromic pigments are compounds containing transition elements, such as vanadium, iron, cobalt, nickel, copper and chromium.
One particular thermochromic pigment which changes from yellow to red at about 300.degree. C. is hydrated iron oxide of the general formula Fe.sub.2 O.sub.3.H.sub.2 O. Thermochromic paints formulated with such a pigment, so as to change color at this temperature, have been found suitable for indicating when a plastic has been sufficiently heated to cause melting of a hot melt adhesive on its inner surface, yet not heated to such a degree to cause permanent degradation of the plastic.
Unfortunately, the use of thermochromic paints containing transition metal thermochromic pigments are in many instances themselves detrimental to the plastic upon which they are coated. More specifically, I have found that polyolefin materials, such as polyethylene, polyethylene co-polymers or terpolymers including ethylene/propylene, ethylene/ethylacrylate, ethylene/vinyl acetate or ethylene/propylene rubber may be adversely affected when coated with such paints and then heated. I have found that such plastics, when heated, tend to glow in spots, and to crater, and ultimately split when coated with paints containing such thermochromic pigments. In comparison, non-coated plastics heated to the same temperature do not degrade as readily. In effect, the thermochromic paint, which has been applied to prevent thermal degradation and excess heating, has been found to contribute to degradation in plastics sensitive to thermo-oxidative degradation.
Although the interreaction of the plastic and the thermochromic pigment is not fully understood, in the case of yellow iron oxide, upon heating, the following reaction is believed to take place: EQU Fe.sub.2 O.sub.3 .multidot.H.sub.2 O.increment.Fe.sub.2 O.sub.3 +H.sub.2 O EQU Fe.sub.2 O.sub.3 +FeO.increment.Fe.sub.3 O.sub.4
As the heating progresses, the water of hydration is lost and the yellow color darkens to a red. The Fe.sub.2 O.sub.3 remaining is believed to absorb radiant energy causing hot spots on the sleeve, manifesting itself in a glowing phenomenon. In addition, it is hypothesized that certain thermochromic pigments, containing transition metals, may react with the polyolefin plastic in some way, to catalyze the oxidation and degradation of the polyolefin plastic.
It is thus an object of this invention to provide an improved thermochromic paint which will indicate the temperature to which a plastic is heated, but will not contribute to the degradation of that plastic.
It is a further object of this invention, to provide a plastic article, whose temperature can be monitored so it can be heat processed under field conditions where precise heating methods are not available. It is a further object of the present invention to provide a thermochromic paint which will protect a plastic against degradation when it is heated to cause recovery over a substrate.